Two New Mixed-Valence Manganese Complexes of Formula [Mn₄O₂(X-benzoato)₇(bpy)₂] (X = 2-Cl, 2-Br) and the Crystal Structure of the 2-Cl Complex: Ground-State Spin Variability in the [Mn₄O₂]⁷⁺ Complexes

The reaction of [Mn3O(2-X-benzoato)6L3] (X = Cl, Br; L = pyridine) with 2,2‘-bipyridine in CH2Cl2 leads to the high-yield formation of new mixed-valence tetranuclear MnIIMn3III complexes of general formulation [Mn4O2(X-benzoato)7(bpy)2] (1, X = 2-chloro; 2, X = 2-bromo). The crystal structure of 1 was determined. Complex 1 crystallizes in the monoclinic system, space group P21/n with a = 19.849(8) Å, b = 13.908(5) Å, c = 30.722(19) Å, β = 107.35(2)°, Z = 4. Complex 1 is neutral, and consideration of overall charge necessitates a mixed-valence MnIIMnIII3 description. Each manganese ion is distorted octahedral, especially the three MnIII ions, owing to a first-order Jahn−Teller effect. The MnII is assigned on the basis of the longer metal−ligand distances. Variable temperature magnetic susceptibility studies were performed on 1 and 2 in the temperature range 2−300 K. The topology of the molecule requires three J values, Jbb between the two-body MnIII ions and two Jwb (“wing-body”) between the MnIII ions of the “body” of the butterfly and the MnII or MnIII of the “wing” of the butterfly. Without any simplifying assumptions, a full diagonalization matrix method is necessary to solve the problem, but assuming that both Jwb are identical, it is then possible to solve the problem numerically by applying the Kambe method. With both methods, the derived Jbb and Jwb exchange parameters are very similar for the 2-Cl and 2-Br complexes. The best R factors [∑i(χMcalc − χMobs)2/∑i(χMobs)2] (∼10-6) were obtained from 300 to 40 K. The J values are, thus, as follows. For 1, Jbb = −23.2 cm-1, Jwb = −4.9 and −4.8 cm-1, and g = 1.93. For 2, Jbb= −22.8 cm-1, Jwb = −4.8 and −4.7 cm-1, and g = 1.92. With these values, the expected ground-state spin must be 7/2, very close in energy to low-lying spin states of 9/2, 5/2, 3/2, and 1/2. They are all almost degenerate. By application of Kambe's method (with only one Jwb), the results are completely similar. Magnetization measurements at 2−30 K from 2 to 50 kG confirm that the ground state is S = 7/2 for 1, with the D parameter equal to −0.60 cm-1.将[Mn₃O(2-X-苯甲酸根)₆L₃]（X=Cl、Br；L=吡啶(pyridine)）与2,2'-联吡啶(2,2‘-bipyridine)在二氯甲烷(CH₂Cl₂)中反应，可高产率得到通式为[Mn₄O₂(X-苯甲酸根)₇(bpy)₂]的新型混合价态四核Mn(II)Mn₃(III)配合物（配合物1：X=2-氯；配合物2：X=2-溴）。 配合物1的晶体结构已得到解析。 配合物1属单斜晶系(monoclinic system)，空间群P2₁/n，晶胞参数为a=19.849(8) Å，b=13.908(5) Å，c=30.722(19) Å，β=107.35(2)°，Z=4。 配合物1为中性分子，结合整体电荷可推断其为混合价态Mn(II)Mn₃(III)结构。由于一级姜-泰勒(Jahn−Teller)效应，每个锰离子均呈现畸变八面体配位构型，其中三个Mn(III)离子的畸变程度更为显著；Mn(II)离子可通过更长的金属-配体键长予以指认。 在2~300 K的温度范围内，对配合物1和2开展了变温磁化率(magnetic susceptibility)测试。该分子的磁拓扑结构需要引入三个磁交换耦合常数：两个Mn(III)离子之间的Jbb，以及“蝶形”结构中“体部”Mn(III)与“翼部”Mn(II)/Mn(III)之间的两个Jwb（翼-体）耦合常数。若无任何简化假设，需采用完全对角化矩阵(diagonalization matrix)法求解该磁耦合问题；若假设两个Jwb数值相等，则可通过坎贝(Kambe)法进行数值求解。两种方法得到的2-氯和2-溴配合物的Jbb与Jwb交换参数均极为相近。 在300~40 K的温度区间内获得了最优R因子(R factor)[∑ᵢ(χ_M^calc − χ_M^obs)²/∑ᵢ(χ_M^obs)²]（约10⁻⁶）。据此得到的磁耦合参数如下：对于配合物1，Jbb=−23.2 cm⁻¹，Jwb=−4.9和−4.8 cm⁻¹，g因子g=1.93；对于配合物2，Jbb=−22.8 cm⁻¹，Jwb=−4.8和−4.7 cm⁻¹，g因子g=1.92。基于上述参数，配合物的基态自旋(ground-state spin)应为7/2，其能量与低激发自旋态9/2、5/2、3/2及1/2极为接近，几乎简并。 采用仅含单一Jwb的坎贝法计算得到的结果与上述完全一致。在2~30 K、外磁场2~50 kG条件下开展的磁化强度测试证实，配合物1的基态自旋S=7/2，零场分裂D参数(D parameter)为−0.60 cm⁻¹。